Deep well structures with single depth shallow trench isolation regions

ABSTRACT

A method of forming a semiconductor device includes defining a first type region and a second type region in a substrate, t separated by one or more inter-well STI structures; etching and filling, in at least one of the first type region and the second type region, one or more intra-well STI structures for isolating semiconductor devices formed within a same polarity well, wherein the one or more inter-well STI structures are formed at a substantially same depth with respect to the one or more intra-well STI structures; implanting, a main well region, wherein a bottom of the main well region is disposed above a bottom of the one or more inter-well and intra-well STI features; and implanting, one or more deep well regions that couple main well regions, wherein the one or more deep well regions are spaced away from the one or more inter-well STI structures.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.12/691,196, filed Jan. 21, 2010, the disclosure of which is incorporatedby reference herein in its entirety.

BACKGROUND

The present invention relates generally to integrated circuit (IC)device fabrication and, more particularly, to deep well structures withsingle depth, inter-well and intra-well shallow trench isolationregions.

A typical semiconductor device in a complementarymetal-oxide-semiconductor (CMOS) circuit is formed in a p-well or ann-well in a semiconductor substrate. Since other semiconductor devicesare also present in the semiconductor substrate, a given semiconductordevice requires electrical isolation from adjacent semiconductordevices. Electrical isolation is provided by isolation structures thatemploy trenches filled with an insulator material (e.g., shallow trenchisolation or “STI” regions). The electrical isolation of a semiconductordevice from other devices located in the same well is referred to as“intra-well” isolation. In contrast, the electrical isolation of asemiconductor device from other devices in an adjacent well (typicallyof the opposite polarity type, but could also be a same polarity typewell of a different well bias) is referred to as “inter-well” isolation.In either case, the unintended functionality of parasitic devices, suchas parasitic p-n-p or n-p-n bipolar transistors, defined by variouselements of the semiconductor device and adjacent semiconductor devices,needs to be suppressed. This is typically done by placing a dielectricmaterial, such as an STI structure, in the current paths of the elementsof the parasitic devices.

However, electrical isolation between n-well and p-well regions in bulkCMOS technologies presents somewhat of a quandary. On the one hand,leakage current between n-wells and adjacent p-wells is minimized (andlatchup parameters are improved) by having STI features that penetrateall the way through the bottoms of the highly doped (deep) well regions.These deep STI features force potential latchup currents deep into thesubstrate and separate active regions in adjacent oppositely dopedwells, thus providing good inter-well isolation. On the other hand, anSTI must be sufficiently shallow so that distinct devices within thesame polarity well (e.g., an n-well) are not cut off from one other.That is, for intra-well isolation, the STI must be shallower than thebottom of the doped well to also achieve reasonable intra-wellresistances.

Perhaps the most commonly suggested method for eliminating this quandaryis to use two separate STI depths, one for inter-well isolation andanother for intra-well isolation. More specifically, shallow STI (i.e.,“shallow shallow trench isolation”) is used between devices within thesame well for intra-well isolation, while deep STI (i.e., “deep shallowtrench isolation”) is used between wells for inter-well isolation.Unfortunately, problems with this dual-depth STI approach include thecost of an additional critical STI mask and etch, as well as thedifficulty in etching and filling the higher aspect ratio trenches thatresult (e.g., aspect ratios of >10:1 for 22 nanometer deep STI). Inaddition, the lateral scattering and diffusion of deep well dopants,typically added to wells via high-energy implants, as well as potentialmisalignment of masks used to define the well's positions may also limitthe minimum n+ to p+ spacing to much wider dimensions than arecompatible with aggressively scaled SRAM (and other circuit device)features.

SUMMARY

In an exemplary embodiment, a method of forming a semiconductor deviceincludes defining a first type region and a second type region in asubstrate, the first type region and second type region separated by oneor more inter-well shallow trench isolation (STI) structures; etchingand filling, in at least one of the first type region and the secondtype region, one or more intra-well STI structures for isolatingsemiconductor devices formed within a same polarity well, wherein theone or more inter-well STI structures are formed at a substantially samedepth with respect to the one or more intra-well STI structures;implanting, in the at least one of the first type region and the secondtype region, a main well region, wherein a bottom of the main wellregion is disposed above a bottom of the one or more inter-well andintra-well STI features; and implanting, in the at least one of thefirst type region and the second type region, one or more deep wellregions that couple main well regions otherwise isolated by the one ormore intra-well STI structures, wherein the one or more deep wellregions are spaced away from the one or more inter-well STI structures.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Referring to the exemplary drawings wherein like elements are numberedalike in the several Figures:

FIG. 1 is a cross-sectional view of a pair of trenches formed in a CMOSsubstrate, one of which serves as an inter-well STI and the other ofwhich serves as an intra-well STI, suitable for use in accordance withan embodiment of the invention;

FIG. 2 illustrates an example of lateral bulging of a conventionallyformed deep n-well implant of the substrate of FIG. 1;

FIG. 3 is a cross-sectional view of first shallow well implant and asecond deep well implant formed in the substrate of FIG. 1, inaccordance with an embodiment of the invention;

FIG. 4 illustrates an alternative embodiment of the second deep wellimplant structure of FIG. 3; and

FIG. 5 illustrates another alternative embodiment of the second deepwell implant structure of FIGS. 3 and 4.

DETAILED DESCRIPTION

Disclosed herein is a semiconductor device providing deep well implantstructures with single depth, inter-well and intra-well shallow trenchisolation regions. Generally stated, a single STI depth is utilized incombination with doping the well(s) twice—once with a relatively shallowdopant profile whose bottom is above the STI depth, and second with adeeper dopant profile that is deeper than the STI depth. The lateralextent of the shallow dopant profile encompasses the entirety of thewell boundaries, while the lateral extent of the deeper dopant profileis located substantially throughout the well, except for the regionsadjacent to wells of the opposite polarity type. In so doing, the deeperdopant profile electrically connects same polarity type well portionsseparated by intra-well STIs, while the shallow dopant profile improveslatchup and sets desired device behavior for individual devices formedat the well surfaces.

If the substrate is p-type, then it is conceivable that only the n-wellsmay require the deep profile; the shallow profile alone may suffice forp-wells because substrate doping will connect p-wells separated by STIs.Regardless, a principal advantage of the presently disclosed embodimentsis that it is a relatively simple process, avoiding the need fordual-depth STI formation and the attendant problems associatedtherewith. Moreover, simple block masks can be used for the deep dopantprofiles. Furthermore, since the deep well profiles are not immediatelyadjacent the border between p-wells and n-wells, problems with minimumn+ to p+ spacing discussed above should be much reduced.

As illustrated in further detail below, one particular exemplaryembodiment utilizes masked implants to form the shallow n-well andp-well profiles. These shallow implants are above the (single) STIdepth, to the point where the implanted well(s) are completely cut offby the STI. Then, one or more second well masks are utilized to add deepimplants to electrically connect adjacent wells of the same doping type.This deep doping would penetrate below the depth of the STI, but wouldbe sufficiently removed from the oppositely doped well regions and fromborders therebetween.

For example, one exemplary process flow would remove the deepest n-wellimplants from the n-well implant sequence (and other n-well implantsthat penetrate below the STI depth). Then a second n-well mask isemployed to add desired deep well implants to connect devices residingcompletely within the same n-well. These deep implants would be maskedso that they stay away from closely spaced p-well regions. In addition,p-well processing (well implants and masks) would be adjusted to providesufficient p-type substrate conductivity and latchup resistance.

In another contemplated embodiment, the deep dopant profile(s) isdefined prior to filling the isolation trenches. The additional maskingstep(s) is performed just after STI reactive ion etching (RIE) or afterSTI sidewall passivation. Similar to the first embodiment, an (added)mask protects borders between opposite polarity wells (and oppositelydoped wells), and defines regions into which shallow doping profiles areintroduced. This shallow doping can be performed using ion implantationat sufficiently low energy so that a pad nitride absorbs ions that wouldotherwise affect active regions, or via direction “infusion” (clusterbeam doping or other variations). The specific embodiments have theadvantage of utilizing well-known process techniques and relativesimplicity compared to dual-depth STI.

Referring initially to FIG. 1, there is shown cross-sectional view of aCMOS substrate 100 having a pair of trenches 102, 104 formed thereinsuitable for use in accordance with an embodiment of the invention. Inthe example depicted, region 106 represents an NMOS area of thesubstrate 100, while region 108 represents a PMOS area of the substrate100. Thus, trench 102 serves as an inter-well STI feature (bordering theNMOS area 106 and PMOS area 108), while trench 104 serves as anintra-well STI feature (being completely within the PMOS area 108). Boththe trench 102 and trench 104 are formed at substantially the same depthand have similar minimum widths, in silicon substrate 100. In anillustrative embodiment, the exemplary dimensions (e.g., aspect ratioapproximately 5:1 or more) shown in FIG. 1 may represent a 22 nanometer(nm) node. Also depicted for purposes of illustration in FIG. 1 are anN+ area 110 within the NMOS area 106 and a P+ area 112 within the PMOSarea 108, that are typically formed subsequent to well formation, butare nonetheless shown for later reference.

FIG. 2 illustrates an outline of a typical n-well profile, in this caseintended to be perfectly aligned with the center of the inter-well STIfeature 102. However, as a practical matter, the actual well profiletypically exhibits a bulge 202 therein due to lateral straggle of thehigh-energy implants needed to achieve low-resistivity doping below thebottoms of the STI features 102, 104. The N+ to n-well distance 204 is acritical parameter in determining (N+) junction leakage, and can becomedangerously small as the n-well 200 misaligns to the left or whenimplant straggle forms n-well pockets on the p-well-facing surface ofinter-well STI feature 102.

Accordingly, FIG. 3 is a cross-sectional view of a first shallow wellimplant and a second deep well implant formed in the substrate of FIG.1, in accordance with an embodiment of the invention. More specifically,a main n-well implant region 300 is defined so that the bottom of themain n-well region 300 lies above the bottom of both the inter-well STI102 and the intra-well STI 104. As indicated above, the lateral extentof the main n-well region 300 (and main wells in general) encompassesthe entirety of the well boundaries. That is, the main well region 300abuts the inter-well STI features 102 so as to fully occupy activedevice regions.

In addition, adjacent n-well regions 300 isolated by the deeperintra-well STIs 104 are connected by a deep, separately masked implantedregion 301, so that well contacts are not required for each n-well. Thedeep implant region 301 is patterned so that (unlike the main implantregion 300) it does not abut the n-well/p-well border and is adequatelyspaced away from the inter-well STI region 102. As will further be notedfrom FIG. 3, any bulges 302 in the main n-well 300 profile due tolateral implant scatter, diffusion, or overlay tolerance will notundercut the inter-well STI regions 102 separating adjacent n- andp-well (or adjacent separately biased n-wells) so N+/n-well leakage isnot degraded via that mechanism. However, if the bulge 301 from themedium-depth n-well implants should happen to misalign and/or scatterinto a section of an adjacent p-well, it will be isolated from anyn-well so that will not contribute to leakage.

With the above described approach, the additional (non-critical) blockmask for forming the second, deep implant region 301 will be much lessexpensive than forming two separate STI depths, and employs existingCMOS technologies.

In an alternative embodiment, it is also contemplated that the deep wellimplant regions for coupling same polarity well regions may also beformed while the STI trenches are still open (e.g., immediately aftersidewall oxidation/passivation). In this case, masked shallow implants(or directional “infusions”) of appropriate dopant are formed within theSTI bottoms in order to connect adjacent same polarity wells.

In this regard, FIG. 4 is a cross-sectional view of a substrate 400featuring an alternative embodiment of the second, deep well implantstructure of FIG. 3. Similar to FIG. 3, the substrate 400 includes anNMOS area 402 and a PMOS area 404 separated from one another by aninter-well STI 406. Further, the NMOS area 402 also includes intra-wellSTIs 408 while the PMOS area 404 includes intra-well STIs 410. Again, itwill be noted that depth of the both the inter-well STI 406 andintra-well STIs 408, 410 extends below the main well implant regions(i.e., p-well 412 and n-well 414). That is, inter-well STI 406completely isolates adjacent p-wells 412 and n-wells 414. Here, thesecond implant structures are formed while the trenches of intra-wellSTIs 408, 410 are still open. For example, with a pad nitride (notshown) in place, the bottoms of the open intra-well STIs are subjectedto shallow ion implantation (I/I) operations so as to define dopedregions 416 and 418. The doped regions 416 connect adjacent p-wellregions 412 and the doped regions 418 connected adjacent n-well regions414. The pad nitride protects active regions of the substrate byabsorbing the shallow I/I.

Finally, FIG. 5 is a cross-sectional view of a substrate 500 thatcombines the concepts of the second deep well implant structures ofFIGS. 3 and 4. For purposes of illustration, similar regions aredesignated with the same reference numbers as in FIG. 4. In addition tothe main p-well and n-well implants 412, 414, the embodiment of FIG. 5incorporates both the masked shallow I/I process of the open trenchbottoms to form doped regions 416, 418 (as in FIG. 4) and a deep implantto form regions 420 and 422 (as in FIG. 3). Similar to the embodiment ofFIG. 3, the deep implant of p-well regions 420 and n-well regions 422 iskept away from the p-well/n-well borders edges through an appropriatemask design, but is deep enough to add well “volume” under the bulk ofthe main wells 412, 414. As will be further noted from FIG. 5, where thedeep implant occurs through open STI trenches, well pockets 420′, 422′will be formed that may or may not actually connect to the main well.Such additional deep well pockets are harmless. In any case, both theintra-well connecting doped regions 416 and the deep well pockets 420′in the NMOS area 402 are doped using the same block mask, and theintra-well connecting doped regions 418 and the deep well pockets 422′in the PMOS area 404 are doped using the same block mask.

Although the exemplary embodiments of inter-well isolation structuresdescribed herein are presented in the context of separating oppositepolarity wells, the principles herein could also be applied toseparating wells of the same polarity, but perhaps at different wellbias levels. For example, an inter-well STI structure could isolatefirst and second n-wells from one another, while an intra-well STIstructure in this context could isolate individual devices within thefirst n-well, for example.

While the invention has been described with reference to a preferredembodiment or embodiments, it will be understood by those skilled in theart that various changes may be made and equivalents may be substitutedfor elements thereof without departing from the scope of the invention.In addition, many modifications may be made to adapt a particularsituation or material to the teachings of the invention withoutdeparting from the essential scope thereof. Therefore, it is intendedthat the invention not be limited to the particular embodiment disclosedas the best mode contemplated for carrying out this invention, but thatthe invention will include all embodiments falling within the scope ofthe appended claims.

What is claimed is:
 1. A method of forming a semiconductor device, themethod comprising: defining a first type region and a second type regionin a substrate, the first type region and second type region separatedby one or more inter-well shallow trench isolation (STI) structures;etching and filling, in at least one of the first type region and thesecond type region, one or more intra-well STI structures for isolatingsemiconductor devices formed within a same well, wherein the one or moreinter-well STI structures are formed at a substantially same depth withrespect to the one or more intra-well STI structures; implanting, in theat least one of the first type region and the second type region, a mainwell region, wherein a bottom of the main well region is disposed abovea bottom of the one or more inter-well and intra-well STI features;implanting, in the at least one of the first type region and the secondtype region, one or more deep well regions that couple main well regionsotherwise isolated by the one or more intra-well STI structures, whereinthe one or more deep well regions are spaced away from the one or moreinter-well STI structures; and forming the one or more deep well regionsby shallow ion implantation through open trenches of the one or moreintra-well STI structures, such that the one or more deep well regionsare localized at sidewall and bottom surfaces of the one or moreintra-well STI structures.
 2. The method of claim 1, further etching andfilling forming the one or more deep well regions using a separate maskwith respect to forming the main well region, such that the one or moredeep well regions are disposed below both main well regions and thebottoms of the one or more intra-well STI structures.
 3. The method ofclaim 1, wherein the first type region comprises a p-type region and thesecond type region comprises an n-type region.
 4. The method of claim 3,wherein the one or more main well regions and deep well regions comprisen-wells formed in the p-type region of the substrate.
 5. The method ofclaim 4, wherein the p-type region comprises a p-type metal oxidesemiconductor (PMOS) region and the n-type region comprises an n-typemetal oxide semiconductor (NMOS) region.
 6. A method of forming asemiconductor device, the method comprising: defining a first typeregion and a second type region in a substrate, the first type regionand second type region separated by one or more inter-well shallowtrench isolation (STI) structures; etching and filling, in at least oneof the first type region and the second type region, one or moreintra-well STI structures for isolating semiconductor devices formedwithin a same well, wherein the one or more inter-well STI structuresare formed at a substantially same depth with respect to the one or moreintra-well STI structures; implanting, in the at least one of the firsttype region and the second type region, a main well region, wherein abottom of the main well region is disposed above a bottom of the one ormore inter-well and intra-well STI features; implanting, in the at leastone of the first type region and the second type region, one or moredeep well regions that couple main well regions otherwise isolated bythe one or more intra-well STI structures, wherein the one or more deepwell regions are spaced away from the one or more inter-well STIstructures; etching and filling the one or more deep well regions usinga separate mask with respect to forming the main well region, such thatthe one or more deep well regions are disposed below both main wellregions and the bottoms of the one or more intra-well STI structures;and performing shallow ion implantation through open trenches of the oneor more intra-well STI structures, such that the one or more deep wellregions are also localized at sidewall and bottom surfaces of the one ormore inter-well STI structures.